Can Humans Be Changing the Climate?

ACS Climate Science Toolkit | Narratives

Remark: “The climate of the Earth is warming. Do you think that human activities could be causing the warming?”

Reply: “The Earth is huge. How is it possible for we humans to have an effect on its climate?”

Humans have been changing the face of the Earth for thousands of years. We developed agriculture that has changed the landscape, clearing trees and other vegetation to make way for our crops. We domesticated and bred animals for food, transportation, and labor, with the parallel production of waste products from herds of these new animals. We formed societies, developed specialized occupations, urbanized, and constructed vast new manmade landscapes quite different from nature. And, in one of our greatest achievements, we conquered the power of fire to drive engines that replaced the labor of animals and people or energy harnessed from moving water. The result, the Industrial Revolution, was a leap forward for lifestyle in some parts of world and desired by most of the rest.

To maintain that lifestyle, we require large amounts of energy to make what we need and want, to light and heat our homes, and to transport us among the places we live, work, and play. The majority of the energy we use comes from burning fossil fuels—coal, oil, or gas (or electricity produced by burning these fuels)—and carbon dioxide gas is a product of this burning. The carbon dioxide goes off into the air and increases the amount that was already there naturally as an important part of the greenhouse effect that keeps the Earth warm enough for life as we know it. A little over 50 years ago, we started measuring the amount of carbon dioxide in the air and the results are shown in this graph.

The grey squiggly curve on this graph plots the monthly average amounts of carbon dioxide in the air at a weather station in Hawaii. (Measurements like these are now being made at many places, but this was the site where the first ones were made.) The amounts are given as ppmv, the number of molecules of carbon dioxide in a sample of one million molecules of dry air. Most of the other molecules are nitrogen and oxygen. The red curve is the yearly average that smooths out the squiggles, the seasonal variations. One such annual variation is shown in the inset at the lower right. These variations occur because green plants use carbon dioxide when they photosynthesize sugars during the spring and summer growing season. From about April to October, carbon dioxide is being removed from the air and then it builds up again during fall and winter, when the plants are not growing.

You can see that the average amount of carbon dioxide in the air increased from about 315 to 385 ppmv from 1957 to 2007 and it has continued to go up to about 396 ppmv in 2012. To put these numbers in perspective, we need to consider how much carbon dioxide was present in the air before we started burning large amounts of fossil fuels. Since we were not measuring the carbon dioxide before 1957, how are we to find samples of air from earlier times? The answer is, “in bubbles of air trapped in ice.”

As part of explorations of the vast ice sheets that cover Antarctica and Greenland, as well as ice caps and glaciers in other places, ice cores have been bored out of these sheets. This is much like you might use an apple corer to remove an apple core before eating or baking the fruit. These cores can be several kilometers deep and are made up of layers of ice that have built up over hundreds of thousands of years of snowfall. As the snow fell, air became trapped within the snow and ultimately ended up as tiny bubbles in the ice, like those shown in the sidebar photo. Samples of the ice from different layers, corresponding to different times before the present, can be melted to release the trapped gases for measurement. This graph shows the results from these analyses compared to the direct measurements in the graph above.

The time axis on the graph starts about 400,000 years ago and extends to the present time. The blue line is a plot of the amount of carbon dioxide in the air trapped in the ice core at a time before now. There are peaks and valleys in the plot that correspond to higher and lower amounts of carbon dioxide in the trapped air. Other measurements from the cores show that the valleys, less carbon dioxide, correspond to ice ages when the Earth was colder than it is now and the peaks, more carbon dioxide, are warmer periods, like now. These variations are consistent with carbon dioxide’s role as a greenhouse gas helping to keep the Earth warm—warmer when there is more carbon dioxide in the air.

The most recent 1000 years are represented by the big jump at the end of the plot that is enclosed by the dashes. That tiny part of the scale is expanded in the inset so you can see that the increase in carbon dioxide begins a little before 1800, in the early days of the Industrial Revolution, and climbs rapidly near the end of the 20th century. The final values on the curve are the direct measurements from the first graph above. For at least 400,000 years, until the 20th century, the amount of carbon dioxide in the air was never higher than 300 ppmv. Now, about 116 ppmv of new carbon dioxide have been added to the air in the past 200 years. This is an about 925 trillion (925 × 1012) extra kilograms of carbon dioxide (equivalent to the weight of about 2.3 billion fully loaded Boeing 747 airliners).

The total amount of carbon dioxide released to the air by fossil fuel burning and other human activities since the beginning of the Industrial Revolution is estimated to be about 1700 trillion kilograms, so somewhat more than half of this is still present in the air. Most of the rest has either dissolved in the oceans or been used in photosynthesis by green plants and microorganisms (algae). The increased carbon dioxide in the air since humans began burning large amounts of fossil fuels is larger than any change for the past hundreds of thousands of years. Humans can indeed have a large impact on Earth’s composition. Since carbon dioxide is a greenhouse gas, its increase has a warming effect, see the narrative, “What Is the Greenhouse Effect?” Human activities are very likely to have contributed to the observed warming of the Earth.